Remove now-unnecessary ensure_unique_type_paths invocation

[jsdw]

We now call this manually in the macro/CLI prior to checking derives/substitutes, so no need to call it again here.

[jsdw]

Actually, a test fails:

    thread 'full_client::codegen::documentation::check_documentation' panicked at testing/integration-tests/src/full_client/codegen/documentation.rs:63:10:
    API generation must be valid: TypeGeneration(DuplicateTypePath("pallet_referenda::pallet::Event"))

This implies that we do actually point directly to the RuntimeGenerator somewhere, or else have missed something. This isn't a very important PR so I'll probably close it for now.